1. Field of the Invention
The present invention relates to a thermal transfer sheet. In particular, the present invention relates to a thermal transfer sheet, in which a binder and spherical particles are used for a heat-resistant lubricating layer.
2. Description of the Related Art
A thermal transfer system by using a sublimation dye transfers a large number of color dots to a transfer receiver through a very short time heating so as to reproduce a full color image based on the color dots of a plurality of colors.
In this thermal transfer system, a so-called sublimation thermal transfer sheet, in which a dye layer composed of a sublimation dye and a binder is disposed on one surface of a base material sheet, e.g., a polyester film, is used as a thermal transfer sheet.
In the thermal transfer system, a thermal transfer sheet is heated from the back with a thermal head in accordance with image information so as to transfer a dye contained in a dye layer to a transfer receiver (photographic paper) and, thereby, form an image.
At this time, regarding the thermal transfer sheet, it is desired that a surface on the side coming into contact with the thermal head stably exhibits low friction over low density image printing to high density image printing. In general, the thermal transfer sheet is provided with a heat-resistant lubricating layer on the surface opposite to the surface, on which the dye layer is disposed, in order to prevent fusion with the thermal head and give smooth running smoothness.
Incidentally, in image printing on the photographic paper by using a thermal transfer sheet, heat is applied to the heat-resistant lubricating layer from the thermal head and, thereby, a dye in the dye layer on the opposite surface is transferred to the photographic paper. The color formation density is proportionate to an amount of heat, and the surface temperature of the thermal head changes by a few hundreds of degrees, correspondingly. Consequently, when the thermal transfer sheet moves on the thermal head, the friction coefficient between the thermal head and the heat-resistant lubricating layer changes easily because of the temperature change. If the friction coefficient between the thermal head and the heat-resistant lubricating layer changes, movement of the thermal transfer sheet at a constant speed becomes difficult and, thereby, it is difficult to obtain a sharp image.
For example, in the case where the friction coefficient is large, movement of the thermal transfer sheet becomes slow temporarily, and the density of merely that portion may become high. That is, so-called sticking (linear variations in image printing) may occur.
In order to prevent this sticking, it is desirable that the friction coefficient is reduced. As for lubricants to reduce the friction coefficient, phosphate esters and fatty acid esters have been used previously, and the phosphate esters and the fatty acid esters have been contained in the heat-resistant lubricating layers (refer to Japanese Unexamined Patent Application Publication No. 10-35122, for example).
Furthermore, regarding the thermal transfer sheet, spherical particles protruding from a heat-resistant lubricating layer surface are added as a filler for the heat-resistant lubricating layer. In the case where unevenness is provided on a surface of the heat-resistant lubricating layer by the spherical particles, the contact area of the thermal transfer sheet and the thermal head is reduced and the sliding on the thermal head is improved.
However, the phosphate esters and the fatty acid esters, which are used frequently in general, are volatilized or decomposed by heat from the thermal head so as to stain the thermal head. If image printing is further conducted repeatedly with this stained thermal head, adhered materials are baked on the thermal head surface. As a result, variations in image printing and the like may occur in the image printing.
Moreover, if image printing is repeated, a paper powder of photographic paper may be accumulated on the thermal head and, as a result, variations in image printing and the like may occur in the image printing.
Methods for solving them include a method, in which a surface of the thermal head is polished by using an inorganic filler or an organic filler.
In the case where abrasives are used, the surface of the thermal head can be cleaned. However, the thermal head in itself is polished and, therefore, an image in image printing may be affected. In addition, in the case where the abrasives are used, an increase in friction occurs and a load to a printer increases.
On the other hand, in consideration of these problems, a method has been proposed, in which spherical particles protruding from a surface of a heat-resistant lubricating layer and fine particles having particle diameters smaller than those of the spherical particles are included in the heat-resistant lubricating layer of a thermal transfer sheet (refer to Japanese Unexamined Patent Application Publication No. 03-65396, for example).
It is possible to conduct image printing while the friction of a thermal transfer sheet is maintained at a low level and a thermal head is cleaned, by using this method.
Japanese Unexamined Patent Application Publication No. 03-65396 describes that the particle diameter of the smaller particles is preferably 0.01 to 0.1 μm. However, in many cases, particles having such small particle diameters have high hardness and, furthermore, if the particle diameter is made too small, the contact surface area with the thermal head increases, so that the surface of the thermal head may be damaged.